The final episode of "Sherlock" Season 3, "His Last Vow," opens with Charles Augustus Magnussen getting grilled for his easy, regular access to UK's Prime Minister. He is a newspaper proprietor, a private individual and a foreign nation. To the grilling committee, it seems inappropriate that this person has such regular access to the PM of the country.

The terrifying villain of the season, Magnussen, has pressure point on each of the person [read the weakest point] who is part of the committee questioning him, including the lady heading the committee, Lady Elizabeth Smallwood. Her pressure point is her husband and his affair with a minor girl. Magnussen kisses and licks Smallwood's face and she cannot stop him because of the pressure point. After pondering over how to stop Magnussen, Ms Smallwood asks her driver to take a turn to Baker Street.

Watson is having his war nightmare while sleeping holding Mary's hand. The couple get-up to open the door for their neighbour. The neighbour's son is missing, again, and she wants John to find him. Mary accompanies her husband in his search for the boy. At the crack-house, John not only finds the boy but also Sherlock. The detective tells Watson and Mary that he was undercover. Watson calls-up Molly because his best friend needs to pee in the jar.

Sherlock is taken to Molly's lab to check his substance abuse level. Molly slaps him hard, more than a couple of times, and asks him to say sorry and not waste his life. Sherlock deduces that since the ring is not on Molly's finger, her engagement is off.

At 221B, Sherlock sees Mycroft waiting for him. Anderson is there, too, in his apartment with a lady assistant. They are checking the place for drugs. Watson notices that his chair is not there. Sherlock stops Mycroft from opening a door of a room. Sherlock reveals that it is a case and the case is Charles Augustus Magnussen. Mycroft tells his little brother that Magnussen is not his business and if he goes against Magnussen, then he will find himself going against his brother.

When only Watson and Sherlock are left in the apartment, Janine, Mary's main bridesmaid, comes out of the closed room, wearing only a shirt. Watson is flabbergasted to see her there and her behaviour suggests that she and Sherlock are in a relationship. A scene also alludes to Sherlock and Janine taking a shower, together.

Sherlock describes the face of Magnussen as a starfish floating -- flat face dead. He tells Watson that no one can turn his stomach like Charles Augustus Magnussen. Watson is more interested in knowing about Sherlock and Janine's relationship status. He sees Janine kissing Sherlock on her way out for work. Sherlock has nothing to tell on that front, but informs Watson that Magnussen is the 'Napoleon of Blackmail' and he uses his power and wealth to get information on every noticeable person in the Western World. This information is Magnussen's critical pressure points. Sherlock also tells Watson about Magnussen's secret vault, an unassailable architecture of forbidden knowledge. It is the greatest repository of sensitive and dangerous information anywhere in the world.

Sherlock's relationship with Janine is just a ruse to get inside Magnussen's office. Janine is his personal assistant. Sherlock uses a ring proposal to get himself and Watson inside the maximum security room, without alarming anyone.

The two find Janine in a pool of blood. Watson attends to her, while Sherlock walks up the stairs and sees Magnussen kneeling down and someone pointing a gun at him. Sherlock thinks it is Lady Elizebth, as wears Clair de Lune perfume, who has come to get her file. To Sherlock's shock, it turns out to be Mary, that too as an assassin. Sherlock pictures her as a liar. The signs were all there but he chose to not pay close attention to them, including incredible memory, swift reaction time and mystery surrounding her past.

Sherlock and Mary do not get to talk much, as Mary shoots at Sherlock's chest. Sherlock's mind-palace keeps him from fainting soon. Molly, Mycroft, Sherlock's old dog and Moriarty show-up in that mind-palace to keep Sherlock alive. Watson finds him. Moriarty again visits Sherlock in the mind-palace when he is lying unconscious in the hospital bed. Sherlock shows signs of consciousness when he hears Moriarty saying that wife, I worry about. "John Watson is definitely in danger."

Mary visits Sherlock and tells him not to tell anything to Watson. Magnusse opens a cabinet and takes out a file on Mary. He calls her a "bad, bad girl." Janine is safe. She is taking her sweet little revenge on Sherlock by selling to tabloids, spicy, fictitious stories on her fake-relationship with him. Watson and Lestrade visit Sherlock at the hospital, only to find that Sherlock is not in his bed. A hunt is on to find where Sherlock has gone.

Watson and Lestrade question Ms Hudson at 221 B. Watson tells Lestrade that Sherlock knows who shot him. According to Lestrade, if Sherlock is not telling them, it means, he is tracking it down solo. Watson does not agree, and feels he is protecting someone but Sherlock is not someone to care, or protect. Watson sees that his chair is back in the room, which makes him wonder why Sherlock thinks he will come back to Baker Street, and he sees bottle of Claire De Lune on a side table. It is the perfume that Mary also uses.

Mary meets Sherlock at Leinster Garden. Sherlock deduces that Mary shooting at him was not a miss but a surgery. She never intended to kill him. The shot was enough to hospitalise Sherlock, not enough to kill him. It turns out she was a CIA agent and an assassin. She took a new name and a new life when she moved to England. She had intended to kill Magnussen because he knew her real identity and had highly-sensitive information on her, which she did not want Watson to find out. The information has the potential to efface Watson's love for Mary. Sherlock asks Mary, why she did not come to him for help.

At 221 B, a furious and hurt Watson asks Mary to sit in the client's chair and tell about her case, as she is their client now. She hands over a USB flash drive, which has A.G.R.A written on it. She tells Watson all her information is on it and she has just one request. She wants Watson to watch it when she is not around, as he will not love her when he is done watching it and she does not want that to happen.

There is a time jump and it is Christmas day at Senior Holmes' house. The two brothers are there at their parents' house, and Mary is also there. Watson comes there to see her and says that he has not seen the flash drive and he does not intend to find out about her past, as that is her privilege. He throws the flash drive in the fire. The couple can savour the patch-up only for a brief moment, as Mary collapses. Watson finds out that Sherlock has drugged everyone and they are on their way to meet Magnussen, with Mycroft's laptop.

Sherlock had offered to give-up his brother in exchange for getting access to Magnussen's secret vault. At the adversary's place, Sherlock and Watson discover that secret vault does not exist. Magnussen also has a mind-palace like Sherlock and all the information is in his mind. He tells Sherlock and Watson that knowledge is everything, and "knowing is owing." To Watson, Magnussen tells that he knows where to find the people who hate his wife, and he has their numbers. He explains that he will tear his whole life down unless he lets him flick his face, and he does that and later flicks on his head and asks Watson to keep his eyes, open. A helpless Sherlock watches and asks Watson to let Magnussen do what he wishes to do.

Mycroft arrives in a chopper and a team of special force is already on the ground. Magnussen set a trap and Sherlock fell right into it. Magnussen is framing Sherlock and Watson for selling highly confidential state secrets. Before Mycroft's chopper can land, Sherlock shoots down Magnussen and kills him, after saying "you're forgetting. I'm a high-functioning sociopath." Sherlock tells Watson to tell Mary that she is safe now.

As punishment for the murder, Sherlock is being sent off to East Europe to work on a case, under-cover, for six months. It is part of a deal that Mycroft negotiated with Lady Smallwood, saying that there will always come a time when they need Sherlock Holmes and that incarcerating Sherlock will create ruckus.

At the runway, Sherlock rattles off William, Sherlock, Scott, Holmes, suggesting names to Watson for his unborn son. Watson says that it might be a daughter, Sherlock quips that Sherlock is actually a girl's name. The two best-friends say goodbye, not knowing when they will see each other, again.

The private plane, with Sherlock in it, takes-off. However, Mycroft calls him only a few minutes, later, asking him to come back from his exile, as England needs him. Prof. Moriarty is back on every screen in the country in the form of an animated gif, saying the words: "Did you miss me?"

It is a hard-to-believe development. Watson sees Sherlock's plane coming back and he says, "if he's back [Moriarty], he better wrap-up warm, because there is an East Wind coming."